Transdermal drug delivery devices are well known in the pharmaceutical arts as a method for delivering a wide variety of drugs, such as nitroglycerin, estradiol, testosterone, fentanyl, and clonidine. Transdermal drug delivery devices typically involve a carrier (such as a liquid, gel, or solid matrix, or a pressure sensitive adhesive) into which the drug to be delivered is incorporated. Because the skin presents a substantial barrier to ingress of foreign substances into the body, it is often desirable or necessary to incorporate excipients into the carrier that enhance the rate at which the drug passes through the skin. Devices known to the art include reservoir type devices involving membranes that control the rate of drug and/or skin penetration enhancer delivery to the skin, single layer devices involving a dispersion or solution of drug and excipients in a pressure sensitive adhesive matrix, and more complex multilaminate devices involving several distinct layers, e.g., layers for containing drug, for containing skin penetration enhancer, for controlling the rate of release of the drug and/or skin penetration enhancer, and for attaching the device to the skin.
Typical reservoir devices contain a drug in a fluid carrier in the reservoir. The drug diffuses across a membrane to provide controlled release. Among the disadvantages of reservoir type devices are: the complexity of construction and manufacture; large size of the patch (area and thickness); lack of patient comfort; and unattractiveness.
Single layer devices where the drug is directly dispersed or dissolved in the adhesive layer overcome many of the disadvantages associated with reservoir devices. A major disadvantage of single layer devices, however, is the loss of thermodynamic driving force for the drug as the device becomes depleted of drug during the wear period. This loss of driving force causes a decrease in drug delivery rate and often results in a large excess of drug remaining in the device, which is discarded at the end of the wear period.